Field of the Invention
The present invention relates generally to methods and apparatus for processing interference due to passive non-linear products of transmitted signals in a wireless network, and more specifically, but not exclusively, to reduction of interference caused to a receiver due to passive intermodulation (PIM) products generated from at least a first Multiple Input Multiple Output (MIMO) signal comprising first and second MIMO component streams at a first carrier frequency. In other scenarios, there may be two or more carrier frequencies combining to cause PIM, and each carrier frequency may have two or more MIMO component streams.
Description of the Related Technology
Non-linear products may be generated in a wireless network when one or more signals are transmitted along a signal path including a component having a non-linear transmission characteristic; these products typically differ in frequency from the signal or signals from which they were generated, and may potentially cause interference to other signals. The generation of non-linear products is becoming a problem of increasing importance in modern wireless communication systems, and in particular cellular wireless systems, since the radio frequency spectrum available has been steadily expanded as additional bands have become available, and the pattern of allocation of uplink and downlink bands within the available spectrum for use by various cellular systems, such systems using GERAN (GSM EDGE Radio Access Network), UTRAN (UMTS Terrestrial Radio Access Network) and E-UTRAN (Evolved UMTS Terrestrial Radio Access Network) radio access networks, and by various operators, is complex and territorially dependent. In this environment, and in particular at a cellular radio base station, it is likely that circumstances arise in which non-linear products generated from transmitted carriers in one or more downlink bands would fall within an uplink band in which signals are received at the base station. Non-linear products generated by non-linear characteristics of active components such as power amplifiers may generally be dealt with at a design stage by appropriate frequency management and filtering, but non-linear products caused by non-linear characteristics of passive components, for example passive intermodulation (PIM) products, may prove more difficult to manage. Many passive components may exhibit a non-linear transmission characteristic to some degree, for example due to an oxide layer at a metal to metal contact, and the non-linear characteristic may develop with time as an ageing process of the component. For example antenna components, which are external to a receiver but connected to it, may generate PIM to some extent. Furthermore, PIM may be caused by components in the signal path between the transmitter and the receiver that are external to the transceiver equipment and which may be outside the operator's control, such as metallic objects on the antenna tower or other objects in the signal path through the propagation environment, such as fences; this is known as the “rusty bolt” effect.
Increasingly, cellular and other wireless systems use MIMO (Multiple Input, Multiple Output) radio frequency transmission, in which a node such as a base station or a user equipment may have more than one antenna and may transmit and/or receive different signal component streams from each antenna at the same frequency. The node may be in communication with another node which also has more than one antenna and which may also be arranged to transmit and/or receive different signal component streams from each antenna at the same frequency. This may enable an increase in system capacity in some propagation conditions by allowing multiple signal component streams to be transmitted between the nodes at the same frequency. However, if two or more MIMO signal component streams impinge upon one or more passive non-linear devices, then the characteristics of interference received at a receiver can become complicated. It may be desired to generate a replica of the PIM for use in cancelling the PIM interference in a received signal, as for example is described in the applicant's patent GB2502279. However, a simple replica may not be a good representation of the interference received in a complicated interference environment, and as a result cancellation performance may be limited.
It is an object of the invention to address at least some of the limitations of the prior art systems.